A communication system is a facility which facilitates communication between two or more entities such as communication devices, network entities and other nodes. A communication system may be provided by one or more interconnect networks and the elements thereof and a plurality of communication devices, for example user devices. One or more gateway nodes may be provided for interconnecting various networks. For example, a gateway node can be provided between an access network and other communication networks. The communication may comprise, for example, communication of data for carrying communications such as voice, electronic mail (email), text message, multimedia and so on.
A communication system typically operates in accordance with a standard and/or a set of specifications and protocols which set out what the various elements of the system are permitted to do and how that should be achieved. For example, it is typically defined if the user, or more precisely a user device, is provided with a circuit switched bearer or a packet switched bearer, or both. Also, the manner user communication devices can access a communication system is typically defined, as it the manner in which communications should be implemented between the user device and various elements of the communication system. The functions and responsibilities of various entities are also typically defined by communication protocols. Various functions and features are typically, although not necessarily, arranged in a hierarchical or layered structure, so called protocol stack, wherein the higher level layers can influence the operation of the lower level functions.
A user may communicate via a communication system and access various applications by means of an appropriate communication device. The user communication devices are often referred to as user equipment (UE). An appropriate access system allows the communication device to communicate via the communication system. An access to the communications system may be provided by means of a fixed line or wireless communication interface, or a combination of these. Examples of wireless systems include cellular networks, various wireless local area networks (WLANs), wireless personal area networks (WPANs), satellite based communication systems and various combinations of these.
In wireless systems a network entity such as a base station provides an access node for communication devices. It is noted that in certain systems a base station is called ‘Node B’. Typically the operation of a base station node and other apparatus of an access system required for the communication is controlled by an appropriate control entity. The control entity can be interconnected with other control entities of the communication network.
A way of enhancing the coverage and/or throughput of a base station is to use at least one relaying node between the base station and the destined node, for example a destined user device. Relay techniques have been intensively studied, for example, in the context of third and fourth generation communication systems, known in shorthand as 3G/B3G/4G networks. By employing relaying techniques on fixed and mobile relay nodes (RN), the coverage and the throughput of the networks can be increased. Mobile user communication devices can be used as mobile relays to further improve the performance of the networks. In the current cellular network systems a large amount of suitable mobile relay nodes can exist at the same time in a cell. Time division duplex (TDD) mode is often used for relaying communications to the destination device, but this is not the only option.
A scheme known as Inducing Multi-user Diversity Relaying (IMDR) has been proposed for handling the various aspects of relaying by the relay nodes in a cell. The IMDR uses a broadcast feature of a wireless channel to induce multi-user diversity through a two-phase process. In the first phase, known as the feeding phase, data-units are broadcasted by a base station (BS) with its maximum bit-rate and transmit power. It is assumed that at least some user devices in the cell coverage area are likely to receive these data-units. These user devices can then act as mobile relays in the second phase, known as the delivery phase. All user devices which receive a data-unit in the feeding phase act as a relay in the delivery phase. In some disclosures a reference is also made to a three-stage scheme, where a channel quality indicator (CQI) probing phase is provided between the feeding phase and the delivery phase.
It is possible that the destined user device also receives the data directly from the base station. In such a situation the destined user device can send back a positive acknowledgement to the base station. The base station can now determine that the data can be received directly by the destined node without any intermediate i.e. relay nodes. The base station can then broadcast a release command to all relay nodes to instruct the relay nodes to release the relay process. If no positive acknowledgement signalling is received from the destined node, for example during the CQI probing phase, the base station does nothing but is kept inactive. At this stage the relay nodes need to find out the destined node and measure the channel to the destined node. Once the destined node is detected, a hand-shaking can occur between each of the relay nodes and the destined node. Thus, in the IMDR each relay node continuously tracks the quality of the wireless link to the neighbouring users and their identity. In this stage, the relay nodes and the destined node each broadcast signals, and complex hand-shaking protocols are set-up between them to assist in establishing the potential cooperative transmission to the destined node.
The relaying nodes/relaying user devices need to wait until the occurrence of a “good channel” to transmit the data-units into the destination. The transmission occurs with the maximum bit-rate. Transmitting to multiple relay nodes in the first phase induces multi-user diversity into the system that can be exploited in the second-phase, hence the name Inducing Multi-user Diversity Relaying (IMDR).
As mentioned, in the delivery phase the base station is kept inactive. Only transmissions that are allowed are from the relay nodes to the destined node. Upon successful transmission, the destined node sends a positive acknowledgement to the base station. Consequently, the base station broadcasts a release signal where after the relay nodes can release that data-unit. If the base station does not receive an acknowledgement that corresponds to a given data-unit in a predefined time interval, that data-unit is considered lost and a release signal is broadcasted. That lost data-unit may be considered for retransmission later on.
Although the above proposed scheme has proven workable and improved the coverage and efficiency, it also introduces certain disadvantages into the system. In particular, it introduces a signalling overhead between the mobile relay nodes. Thus it would be desired to have a scheme where any excessive signalling overhead could be avoided while the throughput and coverage of a base station can be increased by means of relay nodes. In addition, a scheme that is suitable for a fixed relay station scenario or a scenario when only a few relay nodes are available might also be desired in certain applications.